Debris flow fans: process and form

Abstract:

Lithologic, climatic, and tectonic influences on debris-flow-dominated alluvial fan (debris-flow fan) morphology are elucidated through a combination of field mapping, map and aerial photo analysis, and a theoretical analysis of the controls on debris-flow confinement within channels. Fan size in Owens Valley, California, is controlled primarily by source drainage area and local subsidence rate, as seen elsewhere. However, spatial variability in subsidence rates is shown to be important, and may largely control fan area-drainage area relationships. Depositional patterns on debris-flow fans are governed by interactions between debris flows and channels, which are products of fluvial incision and not debris-flow scour. Channel geometry (i.e., size and gradient) is set by competition between fluvial sediment supply, stream power, and the rate of channel filling by debris flows. The channel-debris flow interaction is controlled by channel geometry, flow volume, flow hydrograph form, and flow rheology. The frequency distributions of these debris-flow properties determine fan morphology. I focus on the role of debris-flow rheology, which is sensitive to debris granulometry and water content. Abundance of boulders and debris silt-and-clay content are important, lithologically controlled properties. Hydrologic conditions associated with debris-flow initiation are hypothesized to be the most important climatically controlled variable. Numerical simulation of open-channel flow of Bingham fluids is utilized to quantify lithologically dependent differences in debris-flow rheology and to analyze the channel-debris flow interaction on fans in a range of field settings. Differences in source lithology exert the dominant influence on debris-flow fan morphology in Owens Valley. Debris-flows from the most fines-rich sources (meta-volcanic source rocks) are more viscous at the same water content (i.e., initiated under the same conditions) than granitic-source debris flows and, consequently, are poorly channelized. As a result, massive overbank deposition near the fan head is common on these fans and they develop steeper radial profiles. Similarly, enhanced overbank deposition leads to the formation of prominent channel-margin levees.